[DDFG] Add randc function for random generation of categorical values

[mm-abogdan]

Create a function named randc to generate a dataset of categorical variables with missingness. Follow the general form of randu & randn, witht he following arguments:

• nlevels: Number of different categories
• shape: Same as in randu & randn, including defaults
• missingness: " "
• thr: " "

The dtype arrgument is not necessary here. Create this function within impyute.dataset.base

Be sure that functions accept & return matrices.
Be sure to follow the 4 steps outlined in contributing.md

The below labels are for DDFG (Data Days for Good) participant reference:
Priority: Medium
Difficulty: Medium